The present invention relates to transconductance amplifiers which are used in high frequency signal processing systems specifically, a transconductance amplifier having a linear amplified response and wide operational bandwidth is disclosed.
Solid state transconductance amplifiers are used as a basic circuit element for many types of signal processing devices. For instance, in the implementation of high order active filters, transconductance amplifiers may be used as an integrator. An ideal integrator provides a transfer function which is proportional to the gain of the transconductance amplifier and an integration capacitance terminating the output of the amplifier.
In order to implement signal processing devices such as high order active filters, the transconductance amplifier must provide a high degree of amplitude linearity over the frequency range of interest. Certain applications for processing signals in the 10 MHZ to 1GHZ frequency spectrum require a linearity of up to xe2x88x9280 dBc for third order intermodulation products (IM3) Additionally, to operate as an ideal integrator, the phase response for the transconductance amplifier terminated with an integration capacitor must be maintained at 90xc2x0 over the bandwidth of interest. The requirement for a high linearity, and a flat phase response of substantially 90xc2x0 over the bandwidth of interest, are particularly important when high order active filters requiring high linearity are implemented using such transconductance amplifiers.
A highly linear and phase compensated transconductance amplifier implemented from a differential amplifier circuit is provided for by the invention. The differential amplifier circuit has first and second bipolar transistors which receive a differential signal for amplification. In order to improve the linearity of the gain response of the differential amplifier circuits, first and second input transistors comprise an input stage to each of the differential amplifier input transistors. The first and second input transistors supply one-half of the differential signal to a respective base of the differential amplifier circuit transistors, and provide feedback from the emitter of the respective differential circuit transistors to their respective bases. The effect is to improve the overall amplitude response linearity for the transconductance amplifier.
The phase of the resulting transconductance amplifier is compensated so that a phase response of 90xc2x0 is maintained over the bandwidth of interest. First and second capacitors cross couple a portion of each half of the differential signal to the bipolar transistor which is receiving the other half of the differential signal. The first and second capacitors provide a pair of dominant poles in the frequency response of the transconductance amplifier, which dominates the pole produced from parasitic capacitances of the input transistors, and produce a pair of zeroes in the complex frequency plane which essentially cancels the resulting dominant poles. The compensated amplifier has a linear amplitude output and a relatively constant phase over the frequency range of interest.